Precisely. Too many people get on and just want to leech (a few just don't know how to share, but those usually figure it out pretty quickly when asked). What's wrong with not wanting leechers to hog slots for hours upon hours grabbing large files they don't have any intention of sharing back to the community? I'm sure some people are jerks about hoarding their special folders (though I honestly don't know how anyone can possibly monetize it and I've never heard of that happening, that sounds more like paranoia to me), but some people are just tired of those entitled folks who think sharers are there only to provide them with what they want when they want it (and who will curse you out if you dare ask them to actually have a list even if you don't want anything from it).
And it might have absolutely nothing to do with rare vs. not-rare, despite what some people think (some people share small files with everyone but reserve large files for those who've proven they're actually interested in sharing and not just downloading what they can without giving back). No one has an obligation to dump all their bandwidth at hit-and-run downloaders.
My formative years in P2P were spent in a community where everyone helped each other out, were very kind and generous - but one cardinal rule was that You Did Not Leech. You shared something, even if you only had a handful of files, you shared what you could and tried to build a decent list.